Discussion Viruses
eduarmiguelrojas
ch05_lecture_ppt.pptxMicrobiology FUNDAMENTALS A Clinical Approach Third Edition
Marjorie Kelly Cowan
&
Heidi Smith
with
Jennifer Lusk
BSN RN CCRN
©McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or further distribution permitted without the prior written consent of McGraw-Hill Education.
Chapter 5
Viral Structure and Multiplication
©McGraw-Hill Education
Learning Outcomes Section 5.1
Explain what it means when viruses are described as filterable.
Identify better terms for viruses than alive or dead.
©McGraw-Hill Education
The Position of Viruses in the Biological Spectrum
Viruses infect every type of cell, including bacteria, algae, fungi, protozoa, plants, and animals
Seawater can contain 10 million viruses per milliliter
For many years, the cause of viral infections was unknown:
Louis Pasteur hypothesized that rabies was caused by a “living thing” smaller than bacteria
He also proposed the term virus, which is Latin for “poison”
©McGraw-Hill Education
Discovery of Viruses
Ivanovski and Beijerinck showed that a disease in tobacco was caused by a virus.
Loeffler and Frosch discovered an animal virus that causes foot-and-mouth disease in cattle.
Filterable virus:
These early researchers found that when fluids from host organisms passed through porcelain filters designed to trap bacteria, the filtrate remained infectious.
This proved that an infection could be caused by a fluid containing agents smaller than bacteria.
©McGraw-Hill Education
Questions About Viruses Remain
Are they organisms; that is, are they alive?
What role did viruses play in the evolution of life?
What are their distinctive biological characteristics?
How can particles so small, simple, and seemingly insignificant be causing disease and death?
What is the connection between viruses and cancer?
©McGraw-Hill Education
The Viral Debate
Two sides of the debate:
Since viruses are unable to multiply independently from the host cell, they are not living things and should be called infectious molecules
Even though viruses do not exhibit most of the life processes of cells, they can direct them, and thus are certainly more than inert and lifeless molecules
Viruses are better described as active or inactive rather than alive or dead
©McGraw-Hill Education
The Vital Role of Viruses in Evolution
Infect cells and influence their genetic makeup
Shape the way cells, tissues, bacteria, plants, and animals have evolved
8% of the human genome consists of sequences that come from viruses
10 to 20% of bacterial DNA contains viral sequences
Obligate intracellular parasites:
Cannot multiply unless they invade a specific host cell and instruct its genetic and metabolic machinery to make and release new viruses
©McGraw-Hill Education
Properties of Viruses(1)
Are obligate intracellular parasites of bacteria, protozoa, fungi, algae, plants, and animals
Estimated 10 31 virus particles on earth, approximately 10 times the number of bacteria and archaea combined
Are ubiquitous in nature and have had major impact on development of biological life
Are ultramicroscopic in size, ranging from 20 nm up to 1,000 nm (diameter)
Are not cells; structure is very compact and economical
Do not independently fulfill the characteristics of life
Basic structure consists of protein shell (capsid) surrounding nucleic acid core
©McGraw-Hill Education
Properties of Viruses(2)
Nucleic acid can be either DNA or RNA, but not both
Nucleic acids can be double-stranded DNA, single-stranded DNA, single-stranded RNA, or double-stranded RNA
Molecules on virus surfaces give them high specificity for attachment to host cell
Multiply by taking control of host cell’s genetic material and regulating the synthesis and assembly of new viruses
Lack enzymes for most metabolic processes
Lack machinery for synthesizing proteins
©McGraw-Hill Education
How Viruses Are Classified and Named
For many years, animal viruses were classified on the basis of their hosts and the diseases they caused
Newer classification systems emphasize the following:
Hosts and diseases they cause
Structure
Chemical composition
Similarities in genetic makeup
International Committee on the Taxonomy of Viruses:
8 orders and 38 families (another 84 families not yet assigned to any order)
©McGraw-Hill Education
Concept Check (1)
Which of the following best describes viruses?
Heterotrophic
Saprobic
Obligate intracellular parasites
Chemoautotrophic
Photosynthetic
©McGraw-Hill Education
Learning Outcomes Section 5.2
Discuss the size of viruses relative to other microorganisms.
Describe the function and structure(s) of viral capsids.
Distinguish between enveloped and naked viruses.
Explain the importance of viral surface proteins, or spikes.
Diagram the possible nucleic acid configurations that viruses may possess.
©McGraw-Hill Education
Virus Size Range
Smallest infectious agents
Smallest viruses: parvoviruses around 20 nm in diameter
Largest viruses: herpes simplex virus around 150 nm in length
Some cylindrical viruses can be relatively long (800 nm) but are so narrow in diameter (15 nm) that their visibility is limited without an electron microscope
©McGraw-Hill Education
Size Comparison of Viruses with a Eukaryotic Cell (Yeast) and Bacteria
©McGraw-Hill Education
Viral Architecture Is Best Observed with Special Stains and Electron Microscopy
Source: CDCl/Dr. F. A. Murphy (a); ©Phototake (b); ©A.B. Dowsette/SPL/Science Source (c)
©McGraw-Hill Education
Viral Components(1)
Viruses bear no resemblance to cells and lack any of the protein-synthesizing machinery found in cells
Viral structure is composed of regular, repeating subunits that give rise to their crystalline appearance
The structure contains only those parts needed to invade and control a host cell:
External coating
Core containing one or more nucleic acid strains of DNA or RNA
Sometimes one or two enzymes
©McGraw-Hill Education
Viral Components(2)
Capsid: protein shell that surrounds the nucleic acid:
Nucleocapsid: the capsid together with the nucleic acid
Naked viruses consist only of a nucleocapsid.
Envelope: external covering of a capsid, usually a modified piece of the host’s cell membrane
Spikes can be found on naked or enveloped viruses:
Project from the nucleocapsid or the envelope
Allow viruses to dock with host cells
Virion: a fully formed virus that is able to establish an infection in a host cell
©McGraw-Hill Education
Structure of Viruses
©McGraw-Hill Education
Viral Capsid
Capsid:
Most prominent feature of viruses
Constructed from identical protein subunits called capsomeres
Capsomeres spontaneously self-assemble into the finished capsid
Two different types:
Helical
Icosahedral
©McGraw-Hill Education
Viral Envelope
Enveloped viruses:
Take a bit of the cell membrane when they are released from a host cell
Enveloped viruses can bud from:
Cell membrane
Nuclear envelope
Endoplasmic reticulum
More flexible than the capsid so enveloped viruses are pleomorphic
©McGraw-Hill Education
Helical Capsid Structure
| Helical Capsids | The simpler helical capsids have rod-shaped capsomeres that bond together to form a series of hollow discs resembling a bracelet. During the formation of the nucleocapsid, these discs link with other discs to form a continuous helix into which the nucleic acid strand is coiled. |
| Naked | The nucleocapsids of naked helical viruses are very rigid and tightly wound into a cylinder-shaped package. An example is the tobacco mosaic virus, which attacks tobacco leaves. |
| Enveloped | Enveloped helical nucleocapsids are more flexible and tend to be arranged as a looser helix within the envelope. This type of morphology is found in several enveloped human viruses, including influenza, measles, and rabies. |
Naked Capsids
Enveloped Capsids
©Science Source, Source: CDC/Dr. Fred Murphy
©McGraw-Hill Education
Icosahedral Capsid Structure
| Icosahedral Capsids | These capsids form an icosahedron (eye″-koh-suh-hee′-drun)—a three-dimensional, 20-sided figure with 12 evenly spaced corners. The arrangements of the capsomeres vary from one virus to another. Some viruses construct the capsid from a single type of capsomere, while others may contain several types of capsomeres. There are major variations in the number of capsomeres; for example, a poliovirus has 32, and an adenovirus has 252 capsomeres. |
| Naked | Adenovirus is an example of a naked icosahedral virus. In the photo you can clearly see the spikes, some of which have broken off. |
| Enveloped | Two very common viruses, hepatitis B virus and the herpes simplex virus, possess enveloped icosahedrons. |
Naked Capsids
Enveloped Capsids
©Dr. Linda M. Stannard, University of Cape Town/Science Source, ©Dr. Linda M. Stannard, University of Cape Town/Science Source (hep B virus); ©Eye of Science/Science Source
©McGraw-Hill Education
Complex Capsid Structure
| Complex Capsids | Complex capsids, only found in the viruses that infect bacteria, may have multiple types of proteins and take shapes that are not symmetrical. They are never enveloped. The one pictured on the right is a T4 bacteriophage. |
©AmiImages/Science Source
©McGraw-Hill Education
Nucleic Acids: At the Core of a Virus
Genome: the sum total of the genetic information carried by an organism
Viruses contain DNA or RNA, but not both
The number of viral genes is quite small compared with that of a cell:
Four genes in hepatitis B virus
Hundreds of genes in some herpesviruses
Possess only the genes needed to invade host cells and redirect their activity
©McGraw-Hill Education
Variety in Viral Nucleic Acid
DNA viruses: Single-stranded (ss) or double-stranded (ds; linear or circular)
RNA viruses: can be double-stranded, but more often single-stranded:
Positive-sense RNA: ready for immediate translation
Negative-sense RNA: must be converted before translation can occur
Segmented: individual genes exist on separate pieces of RNA
Retroviruses: carry their own enzymes to create DNA out of their RNA
©McGraw-Hill Education
Viral Nucleic Acid
| Virus Name | Disease It Causes | |
| DNA Viruses Examples | ||
| Double-stranded DNA | Variola virus | Smallpox |
| Herpes simplex II | Genital herpes | |
| Single-stranded DNA | Parvovirus | Erythema infectiosum (skin condition) |
| RNA Viruses–Examples | ||
| Single-stranded (+) sense | Poliovirus | Poliomyelitis |
| Single-stranded (−) sense | Influenza virus | Influenza |
| Double-stranded RNA | Rotavirus | Gastroenteritis |
| Single-stranded RNA + reverse transcriptase | HIV | AIDS |
©McGraw-Hill Education
Other Substances in the Virus Particle
Enzymes for specific operations within their host cell:
Polymerases that synthesize DNA and RNA
Replicases that copy RNA
Reverse transcriptase synthesizes DNA from RNA
Completely lack the genes for synthesis of metabolic enzymes
Some viruses carry away substances from their host cell:
Arenaviruses pack along host ribosomes
Retroviruses borrow the host’s tRNA molecules
©McGraw-Hill Education
Concept Check (2)
Which of the following is not a type of viral nucleic acid?
Single-stranded DNA
Double-stranded RNA
Double-stranded DNA
Segmented RNA
All of the types listed are found in viruses.
©McGraw-Hill Education
Learning Outcomes Section 5.3
Diagram the five-step life cycle of animal viruses.
Define the term cytopathic effect and provide one example.
Discuss both persistent and transforming infections.
Provide thorough descriptions of both lysogenic and lytic bacteriophage infections.
©McGraw-Hill Education
Modes of Viral Multiplication
Viruses are minute parasites that seize control of the synthetic and genetic machinery of cells
The nature of the viral replication cycle dictates:
The way the virus is transmitted
What it does to the host
Responses of immune defenses
Human measures to control viral infection
©McGraw-Hill Education
Multiplication Cycles in Animal Viruses
General phases of the animal viral replication cycle:
Adsorption
Penetration
Uncoating
Synthesis
Assembly
Release
The length of the replication cycle varies from 8 hours in polioviruses to 36 hours in herpesviruses
©McGraw-Hill Education
Adsorption
A virus can invade its host cell only through making an exact fit with a specific host molecule
Host range: the limited range of cells that a virus can infect:
Hepatitis B: liver cells of humans
Poliovirus: intestinal and nerve cells of primates
Rabies: various cells of all mammals
Cells that lack compatible virus receptors are resistant to adsorption and invasion by that virus
Tropisms: specificities of viruses for certain tissues
©McGraw-Hill Education
Viral Attachment Process
©McGraw-Hill Education
Penetration and Uncoating
The flexible cell membrane of the host is penetrated by the whole virus or its nucleic acid
Penetration through endocytosis happens when an entire virus is engulfed by the cell and enclosed in a vacuole or vesicle
Direct fusion of the viral envelope with the host cell membrane:
Envelope merges directly with the cell membrane, liberating the nucleocapsid into the cell’s interior
©McGraw-Hill Education
Penetration by Animal Viruses
©McGraw-Hill Education
Synthesis: Replication and Protein Production
DNA viruses:
Enter the host cell’s nucleus and are replicated and assembled there
RNA viruses:
Replicated and assembled in the cytoplasm
Retroviruses turn their RNA genomes into DNA
©McGraw-Hill Education
Life Cycle of dsDNA Viruses
Early phase:
Viral DNA enters the nucleus, where genes are transcribed into a messenger RNA
RNA transcript moves into the cytoplasm to be translated into viral proteins (enzymes) needed to replicate the viral DNA
The host cell’s DNA polymerase is involved in this phase
Late phase:
Parts of the viral genome are transcribed and translated into proteins required to form the capsid and other structures
New viral genomes and capsids are assembled
Mature viruses are released by budding or cell disintegration
©McGraw-Hill Education
Assembly and Release
Assembly: virus is put together using “parts” manufactured during the synthesis process
Release: the number of viruses released by infected cells is variable, controlled by:
Size of the virus
Health of the host cell
Poxvirus-infected cell: 3,000 to 4,000 virions
Poliovirus-infected cell: 100,000 virions
Immense potential for rapid viral proliferation
©McGraw-Hill Education
Maturation and Release of Enveloped Viruses
©Chris Bjornberg/Science Source (b)
©McGraw-Hill Education
Life Cycle of Animal Viruses(1)
Adsorption
The virus encounters a susceptible host cell and adsorbs specifically to receptor sites on the cell membrane
The membrane receptors that viruses attach to are usually proteins that the cell requires for its normal function
Glycoprotein spikes on the envelope (or on the capsid of naked viruses) bind to the cell membrane receptors
Penetration and Uncoating
In this example, the entire virus is engulfed (endocytosed) by the cell and enclosed in a vacuole or vesicle
When enzymes in the vacuole dissolve the envelope and capsid, the virus is said to be uncoated, a process that releases the viral nucleic acid into the cytoplasm
©McGraw-Hill Education
Life Cycle of Animal Viruses(2)
Synthesis: Replication and Protein Production
Viral nucleic acid begins to synthesize the building blocks for new viruses
First, the + ssRNA, which is ready to serve as mRNA, starts being translated into viral proteins, especially those useful for further viral replication
The + strand is then replicated into ssRNA becoming the template for the creation of many new + ssRNAs, used as the viral genomes for new viruses
Additional + ssRNAs are synthesized and used for late-stage mRNAs
Some viruses come equipped with the necessary enzymes for synthesis of viral components; others utilize those of the host
Proteins for the capsid, spikes, and viral enzymes are synthesized on the host’s ribosomes using its amino acids
©McGraw-Hill Education
Life Cycle of Animal Viruses(3)
Assembly
Mature virus particles are constructed from the growing pool of parts
Capsid is first laid down as an empty shell that will serve as a receptacle for the nucleic acid strand
Viral spikes are inserted into the host’s cell membrane so they can be picked up as the virus buds off with its envelope
Release
Assembled viruses leave their host in one of two ways:
Nonenveloped and complex viruses that reach maturation in the cell nucleus or cytoplasm are released when the cell lyses or rupture
Enveloped viruses are liberated by budding from the membranes of the cytoplasm, nucleus, endoplasmic reticulum, or vesicles
During this process, the nucleocapsid binds to the membrane, which curves completely around it and forms a small pouch
Pinching off the pouch releases the virus with its envelope
©McGraw-Hill Education
Damage to the Host Cell
Cytopathic effects (CPEs): virus-induced damage to the cell that alters its microscopic appearance
Types of CPEs include:
Gross changes in shape and size
Development of intracellular changes
Inclusion bodies: compacted masses of viruses or damaged cell organelles in the nucleus and cytoplasm
Syncytia: fusion of multiple damaged host cells into single large cells containing multiple nuclei (giant cells)
Accumulated damage from a virus infection kills most host cells
©McGraw-Hill Education
Cytopathic Changes
Source: CDC (a); Courtesy Massimo Battaglia, INeMM CNR, Rome Italy (b)
©McGraw-Hill Education
Persistent Infections
Some cells maintain a carrier relationship: cell harbors the virus and is not immediately lysed:
Can last from a few weeks to the remainder of the host’s life
Can remain latent in the cytoplasm
Provirus:
Viral DNA incorporated into the DNA of the host
Measles virus
Chronic latent state:
Periodically become activated under the influence of various stimuli
Herpes simplex and herpes zoster viruses
©McGraw-Hill Education
Viruses and Cancer(1)
Experts estimate that 13% of cancers are caused by viruses
Transformation: the effect of oncogenic, or cancer-causing viruses:
Some viruses carry genes that directly cause cancer
Other viruses produce proteins that induce a loss of growth regulation, leading to cancer
©McGraw-Hill Education
Viruses and Cancer(2)
Transformed cells:
Increased rate of growth
Changes in their chromosomes
Changes in cell’s surface molecules
Capacity to divide indefinitely
Oncoviruses: mammalian viruses capable of initiating tumors:
Papillomaviruses
Herpesviruses
Hepatitis B virus
HTLV-I
©McGraw-Hill Education
Viral Induction of Cancer
©McGraw-Hill Education
Viruses That Infect Bacteria
Bacteriophage: “bacteria eating”:
Most contain double-stranded DNA, but some RNA types exist as well
Every bacterial species is parasitized by various specific bacteriophages
The bacteria they infect are often more pathogenic for humans
©McGraw-Hill Education
T-Even Bacteriophage
Infect E. coli
Structure:
Icosahedral capsid containing DNA
Central tube surrounded by a sheath
Collar
Base plate
Tail pins
Fibers
©McGraw-Hill Education
Events in the Lytic Cycle of T-even Bacteriophages(1)
©McGraw-Hill Education
Events in the Lytic Cycle of T-even Bacteriophages(2)
©McGraw-Hill Education
Lysogeny: The Silent Virus Infection
Temperate phages:
Undergo adsorption and penetration
Do not undergo replication or release immediately
Viral DNA enters an inactive prophage state:
Inserted into bacterial chromosome
Copied during normal bacterial cell division
Lysogeny: a condition in which the host chromosome carries bacteriophage DNA
Induction: prophage in a lysogenic cell becomes activated and progresses directly into viral replication and the lytic cycle
©McGraw-Hill Education
The Role of Lysogeny in Human Disease
Occasionally, phage genes in the bacterial chromosome cause the production of toxins or enzymes that the bacterium would not otherwise have
Lysogenic conversion: when a bacterium acquires a new trait from its temperate phage:
Corynebacterium diphtheriae - diphtheria toxin
Vibrio cholerae - cholera toxin
Clostridium botulinum - botulinum toxin
©McGraw-Hill Education
Concept Check (3)
Put the phases of the life cycle of animal viruses in the correct order.
Assembly
Penetration
Release
Adsorption
Synthesis
Uncoating
©McGraw-Hill Education
Learning Outcomes Section 5.4
List the three principal purposes of cultivating viruses.
Describe three ways in which viruses are cultivated.
©McGraw-Hill Education
Techniques in Cultivating and Identifying Animal Viruses
Viruses require living cells as their “medium”:
In vivo: laboratory-bred animals and embryonic bird tissues
In vitro: cell or tissue culture methods
Primary purposes of viral cultivation:
Isolate and identify viruses in clinical specimens
Prepare viruses for vaccines
Do detailed research on viral structure, multiplication cycles, genetics, and effects on host cells
©McGraw-Hill Education
Using Live Animal Inoculation
Specially bred strains of white mice, rats, hamsters, guinea pigs, and rabbits are the usual choices for viral cultivation
Occasionally, invertebrates such as insects or nonhuman primates are used
Because viruses exhibit host specificity, certain animals can propagate viruses more readily than others
©McGraw-Hill Education
Using Bird Embryos
Bird eggs containing embryos:
Intact and self-supporting unit
Sterile environment
Contain their own nourishment
Chicken, duck, and turkey eggs are the most common choices for inoculation
Viruses are injected through the eggshell by drilling a small hole or making a small window
©McGraw-Hill Education
Using Cell (Tissue) Culture Techniques
Isolated animal cells are grown in vitro in cell or tissue culture rather than in an animal or egg
Cell culture, or tissue culture:
Grown in sterile chambers with special media that contain the correct nutrients for cells to survive
Cells form a monolayer, or single, confluent sheet of cells that supports viral multiplication
Allows for the close inspection of culture for signs of infection
©McGraw-Hill Education
Detection of Viral Growth in Culture
Observation of degeneration and lysis of infected cells
Plaques: areas where virus-infected cells have been destroyed show up as clear, well-defined patches in the cell sheet:
Visible manifestation of cytopathic effects (CPEs)
©McGraw-Hill Education
Normal and Infected Cell Culture
Source: Bakonyi T, Lussy H, Weissenböck H, Hornyák A, Nowotny N. Emerging Infectious Diseases, Vol. 11, No. 2, Feb. 2005.
©McGraw-Hill Education
Detection of Bacteriophages
This same technique is used to detect and count bacteriophages:
Plaques develop when the viruses released by an infected host cell radiate out to adjacent host cells
New cells become infected, die and release more viruses, and the process continues
Plaque manifests as a macroscopic, round, clear space that corresponds to areas of dead cells
©McGraw-Hill Education
Concept Check (4)
Which of the following is not an in vivo method of culturing animal viruses?
Embryonated chicken eggs
Guinea pigs
Dog kidney cell culture
White mice
All of the choices are in vivo methods.
©McGraw-Hill Education
Learning Outcomes Section 5.5
Name three noncellular infectious agents besides viruses.
©McGraw-Hill Education
Prions
Composed primarily of protein (no nucleic acid)
Exact mode of infection is still being investigated
Deposited as long protein fibrils in the brain tissue of humans and animals:
Creutzfeldt-Jakob disease: afflicts the central nervous system and causes degeneration and death
Bovine spongiform encephalopathy (“mad cow disease”)
Shy-Drager syndrome or multiple system atrophy resembles Parkinson’s disease
©McGraw-Hill Education
Satellite Viruses
Dependent on other viruses for replication
Adeno-associated virus (AAV):
Originally thought that it could only replicate in cells infected with the adenovirus
Can also infect cells that are infected with other viruses
Delta agent:
Naked circle of RNA
Expressed only in the presence of the hepatitis B virus
Worsens the severity of liver damage
©McGraw-Hill Education
Viroids
Virus-like agents that parasitize plants
About one-tenth the size of an average virus
Composed of naked strands of RNA, lacking a capsid or any other type of coating
Significant pathogens in economically important plants: tomatoes, potatoes, cucumbers, citrus trees, chrysanthemums
©McGraw-Hill Education
Concept Check (5)
Creutzfeldt-Jakob disease and Bovine spongiform encephalopathy are caused by prions. Which of the following best describes a prion?
Viral particle
Naked DNA
Infectious protein
Small bacterium
Naked RNA
©McGraw-Hill Education
Learning Outcomes Section 5.6
Analyze the relative importance of viruses in human infection and disease.
Discuss the primary reason that antiviral drugs are more difficult to design than antibacterial drugs.
©McGraw-Hill Education
Viruses and Human Health
Common causes of acute infections:
Colds, hepatitis, chickenpox, influenza, herpes, warts
Prominent viral infections worldwide:
Dengue fever, Rift Valley fever, yellow fever
Infections with high mortality rates:
Rabies, AIDS, Ebola
Infections that cause long-term disability:
Polio, neonatal rubella
Connection to chronic infections:
Type 1 diabetes, MS, various cancers, Alzheimer’s, obesity
©McGraw-Hill Education
Treatment of Animal Viral Infections
Antibiotics designed to treat bacterial infections have no effect on viruses
Difficult to find drugs that will affect viruses without damaging host cells
Almost all antiviral drugs licensed so far have been designed to target one of the steps in the viral life cycle:
Integrase inhibitor class of HIV drugs interrupts the ability of HIV genetic information to incorporate into the host cell DNA
Easier to develop vaccines to prevent viral diseases
©McGraw-Hill Education
Concept Check (6)
Antibiotics are an effective method for treating viral infections.
True
False
©McGraw-Hill Education
Appendix of Image Long Descriptions
©McGraw-Hill Education
Size Comparison of Viruses With a Eukaryotic Cell (Yeast) and Bacteria - Appendix
The yeast cell is approximately 7 micrometers, E. coli is about 2 micrometers long, Streptococcus is about 1 micrometer, and Rickettsia is about 0.3 micrometers long. Most viruses are smaller than eukaryotic and bacterial cells. The largest in this image is Pandovirus, the same size as Streptococcus bacteria, about 1 micrometer. Mimivirus is 450 nanometers, Herpes simplex virus is 150 nanometers, Rabies virus is 125 nanometers, HIV is 110 nanometers, Influenza virus is 100 nanometers, Adenovirus is 75 nanometers, T2 bacteriophage is 65 nanometers, Polio virus is 30 nanometers, and Yellow fever virus is 22 nanometers. For comparison, a hemoglobin molecule (protein molecule) is 15 nanometers.
©McGraw-Hill Education
Viral Architecture is Best Observed With Special Stains and Electron Microscopy - Appendix
(a) Negative staining ofinfluenza virus revealing details of its outer coat. (b) Positive stain of the Ebola virus, a type of filovirus, so named because of its tendency to form long strands. (c) Shadowcasting image of a vaccinia virus.
©McGraw-Hill Education
Structure of Viruses - Appendix
Shown on the left of this figure, the simplest virus is a naked virus (nucelocapsid), consisting of a geometric capsid assembled around a nucleic acid strand or strands. On the right of the figure, an enveloped virus is composed of a nucleocapsid (as shown on the left) surrounded by a flexible membrane called an envelope. In the figure it is represented as a sphere with special receptor spikes inserted into its surface.
©McGraw-Hill Education
Viral Attachment Process - Appendix
An enveloped coronavirus with prominent spikes. The configuration of the spike has a complementary fit for cell receptors. The process in which the virus lands on the cell and plugs into receptors is termed docking.
©McGraw-Hill Education
Penetration by Animal Viruses - Appendix
Endocytosis (engulfment) and uncoating of a herpesvirus: 1. specific attachment, 2. engulfment, 3. virus in vesicle, and 4 vesicle,, envelope, and capsid break down; uncoating of nucleic acid. Fusion of the cell membrane with the viral envelope: 1. specific attachment, 2. membrane fusion, 3. entry of nucleocapsid, and 4. uncoating of nucleic acid.
©McGraw-Hill Education
Cytopathic Changes - Appendix
(a) Human epithelial cells infected by herpes simplex virus demonstrate giant cells with multiple nuclei. (b) Fluorescent-stained human cells infected with cytomegalovirus showing the inclusion bodies. Both viruses disrupt the cohesive junctions between cells, which would ordinarily be arranged side by side in neat patterns.
©McGraw-Hill Education
Viral Induction of Cancer - Appendix
Some retroviruses: Viral oncogenes incorporate into host cell DNA and produce proteins that lead to uncontrolled cell growth. Other retroviruses: Viral genes affect expression of host oncogene leading to uncontrolled cell growth. DNA tumor viruses: Viral genes directly produce proteins that lead to uncontrolled cell growth.
©McGraw-Hill Education
T-Even Bacteriophage - Appendix
After adsorption, the phage plate becomes embedded in the cell wall and the sheath contracts, pushing the tube through the cell wall and releasing the nucleic acid into the interior of the cell.
©McGraw-Hill Education
Normal and Infected Cell Culture - Appendix
(a) Microscopic view of an undisturbed layer of animal cells. (b) Plaques in the animal cell layer. These are open spaces where cells have been disrupted by viral infection.
©McGraw-Hill Education
